Apparatus for generating and superheating high pressure vapor with double attemperatin thereof



Dec. 24, 1963 c. s. SMITH 3,115,123

APPARATUS FOR GENERATING AND SUPERHEATING HIGH PRESSURE VAPOR WITH DOUBLE ATTEMPERATION THEREOF Original Filed Aug. 25, 1954 2 Sheets-Sheet 1 INVENTOR 264 Charles SSmiifi Fl 6 .4 'A'II'TORNEY Dec. 24, 1963 c. 5. SMITH 3,115,123

APPARATUS FOR GENERATING AND SUPERHEATING HIGH PRESSURE VAPOR WITH DOUBLE ATTEMPERATION THEREOF Original Filed Aug. 25, 1954 2 Sheets-Sheet 2 F I G. 2

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INVENTOR C/mr/es 552221 65 BY I ATTORNEY United States Patent 3,115,123 APPARATUS FOR GENERATHNG AND SUPE HEATING HIGH PRlEdSURE VAPOR WETH DOUBLE ATTEMPERATHON THEREOF Charles S. mith, Westfield, N.J., assignor to The liabcock & Wilcox Company, New York, N.Y., a corporation of New Jersey Continuation of application Ser. No. 452,159, Aug. 25, 1954. This application Feb. 18, 1959, Ser. No. 794,205 7 Claims. (Cl. 122-478) This invention relates to the generation of high pressure vapor [for power generation purposes. It is more particularly involved with such high pressure vapor generation, accompanied by high temperature vapor superheating and the high temperature reheating of the generated vapor in a plural reheat cycle, for better heat rates and improved thermal etliciency of the pertinent power generation. This application is a contmuation of my coperrding application Serial No. 452,159, filed August 25, 1954, now abandoned.

The invention involves, for example, the generation of vapor at high pressures which may be of the order of 2,000 to 2,500 p.s.i., or higher, and the superheating and reheating of the generated vapor to temperatures of the order of 10 50 F., or higher. These results are attained in a vapor generating, superheating, and reheating unit involving two or more vapor reheaters supplying reheated vapor at different pressures to different stages of a prime mover. The furnace from which the furnace gases flow over the superheater elements and the reheat-er elements develops gases at maximum gas temperatures higher than temperatures consonant with the safety and high availability operation of the superheater and the reheaters, the invention involving, in its pertinent aspect, the use of lower temperature recycled heating gases and the mixture of these gases with the furnace gases passing from the combustion zone to the superheater and the reheaters, for the purpose of reducing the temperature of the gases to a safe and optimum degree.

The invention is particularly concerned with the maintaining of optimum superheat and reheat temperatures over a wide range of variation of vapor generating rate, or load. When the vapor is generated in wall tubes subject to the heat of the furnace and the furnace gases pass over a. convection superheater, and over convection reheaters, the rate of furnace firing is decreased in response to decreased vapor demand. This decreases the mass flow of gases over the superheater and the reheaters and results in superheated vapor and reheated vapor temperatures considerably below predetermined optimum values, if no compensating influence is applied. In the combination of this invention there is the further difficulty that the superheater, and each of the reheaters has such a natural inherent tendency as to cause them to depart in different degrees, from a predetermined vapor temperature, as the load, or rate of vapor generation changes. This invention involves differential controls of the heating effectiveness of the reheaters and the superheater to compensate for the above indicated inherent tendencies and to maintain predetermined heated vapor temperatures over a wide range of load.

in one instance, the invention attains the above indicated results by dividing the flow of heating gases, with one division primarily for superheating and the other division primarily for reheating. These divisions of gas flow are differentially controlled so that a greater percentage of the gases passes over the reheater surfaces as the load decreases. Then to compensate for the inherent differential tendencies of the two reheaters, the reheating gas flow is divided between two parallel passes and there is provided additional differential cotnrol (or re-apportioning) of the reheating gas flow between these two 3,ll5,l23 Patented Dec. 24, 1953 to maintain the predetermined the reseparate reheater passes final reheated vapor temperature at the outlets of heaters.

In a more specific sense, the invention involves a vapor generating and vapor heating unit having a small volume furnace fired by the burning of a slag forming fuel at temperatures above the fusion temperatures of the incombustible components of the fuel. This furnace is a slag tap furnace from which the fused or molten slag is withdrawn from the bottom of the furnace. The furnace walls and furnace floor includes vapor generating tubes connected into the vaporizing fluid circulation of the unit. The furnace wall tubes are preferably upright tubes, the upper parts of which, beyond the furnace, are arranged to define a small volume gas mixing chamber and, above that chamber, to define the walls of a superheater gas pass extending toward the top of the unit. Alongside this gas pass and parallel therewith, the upper parts of others of the vapor generating tubes define two parallel reheat gas passes leading to the top of the unit and thence, with the superheater gas pass, to a flue. At the outlets of the gas passes there are arranged three independently operable sets of gas flow regulators. One set regulates the gas flow through the superheater gas pass and the other sets differentially regulate the gas flows through the reheater gas passes.

Some of the furnace wall tubes just beyond the furnace, are bent inwardly from opposite walls of the unit to define cantilever or nose type arches. They also define the opposite gas outlets or gas outlet chambers for a recirculated gas system, and some of the arch portions of these tubes are spaced apart to provide openings through which recirculated gas enters the throat between the arches for mixing with the efllux of combustion products coming directly from the fuel burning means. The gas mixing throat between these arches may be considered as a part of a small volume gas mixing chamber intermediate the furnace and the entrance to the gas passes. The recirculated gases for mixing with the burner efllux in this gas mixing chamber are lower tern *crature gases withdrawn from a position in the gas flow path downstream in a gas flow sense from the reheaters and the superheater and directed by appropriate duct work and a recirculated gas fan to the recirculated gas outlet chambers within the arches.

The invention will be concisely set forth in the appended claims, but for a more complete understanding of the invention, its advantages, and its uses, recourse should be had to the following description which refers to the accompanying drawings.

In the drawings:

FIG. 1 is a sectional elevation of the illustrative vapor generating, superheating, and reheating unit;

FIG. 2 is a somewhat diagrammatic plan of the FIG. 1 unit;

FIG. 3 is a detail horizontal section illustrating the arrangement of the vapor generating wall tubes at the position of the section line 3-3 of FIG. 1, showing an arrangement whereby recirculated gas outlet openings are provided in the nose portions of the arches along the walls of the gas mixing chamber; and

FIG. 4 is a diagrammatic view indicating the application of the double reheaters to a multiple stage steam turbine with which the illustrative unit is associated.

At the lower part of the FIG. 1 unit is a slag tap furnace 1t) fired by the pulverized fuel burners 12. Combustion products from the furnace pass upwardly through the gas mixing throat 14, between the arches 16 and 18 into the gas mixing chamber 20. The gas flow upwardly tom the mixing chamber 20 is divided by an intermediate wall 22. Between this wall and the outer wall 24 there is a superheater gas pass 26, and on the right hand side of the wall 22 and between this wall and the outer wall 28 there are two parallel reheater gas passes 30 and 32, separated by an upright wall 34 (see FIG. 2). Within the reheater gas pass 32 there is a low pressure reheater 38, and within the companion gas pass 30 there is a similar, but high pressure reheater.

Within each of the reheater gas passes 30 and 32 there is a first superheater section the inlet header 42 of which receives saturated steam from the steam and water drum 44. From the outlet header 46 of the superheater sections 40 steam flows through the line 48 to an attemperator 50 and thence through the line 52 to the inlet header 54 of the main or second superheater section 56 which includes a plurality of tube banks. From the outlet header 5% of the superheater section 56 superheated steam flows through an appropriate line 60 to and through a second attemperator 62 and thence through a line 64 to the inlet header 66 of the third superheater section 68. From the outlet header 70 of the third superheater section the superheated steam flows to the inlet of the high pressure stage of the steam turbine 74 which is indicated in FIG. 4.

Within the superheater gas pass 26 and above the superheater section 56 there is an economizer including the banks of tubes 76 and 78 associated with an inlet header 80 and an outlet header 82 from which water flows through appropriate conduits to the water space of the drum 44.

At the top of the unit the division wall 34 separating the reheater gas passes 30 and 32 continues between the upper and lower tube sections and 92 through lateral passes 94 to separate outlets 96 each of which is provided with its own separate set of independently operable gas flow regulators such as indicated by the dampers 98. Directly below these sets of dampers is another set of dampers 100 for regulating the flow through the exit of the superheater gas pass 26.

Heating gases flow from the outlets of the gas passes through appropriate duct work 102 to and through a portion of the regenerative air heater 104. From this air heater the gases pass through an outlet 106 to a stack.

The FIG. 1 unit includes a gas recirculating fan 103 driven by an electric motor 110, with the inlet of the fan communicating with duct work 102 through a lateral opening 112. From the outlet of the fan 108 the recirculating gases pass through appropriate duct work 114 to the recirculated gas outlet chambers 116 and 118, the How of recirculated gas flow through the duct work 114 being regulated by a series of dampers 120. From an inspection of FIG. 2 it will be understood that there is a recirculated gas system at each side of the unit and so that the recirculated gases may be delivered to the opposite ends of the recirculated gas outlet chambers 116 and 118 by the duct components 114A and 114B.

Steam exhausting from the high pressure stage 72 of the turbine 74 (FIG. 4) passes through the line to the inlet headers 132 and 134 of the high pressure reheater 33. From the outlet headers 136 of this reheater steam passes through the line 140 to the inlet of the intermediate stage 142 of the turbine from which the steam is exhausted at a lower pressure and temperature through the line 144. Thence the steam passes through inlet headers (similar to the headers 132 and 134) of the low pressure reheater 146. It then passes through the heating tubes of this reheater and from its outlet headers similar to 136 and then through line 148 to the low pressure turbine stage 149 from which exhaust steam passes to ultimate steam outlet 150 of the turbine.

By way of description of the operation of the FIG. 1 unit it will be assumed that the unit is operating at a control point load, or at full load, delivering 1,000,000 lbs. of steam per hour to the turbine 74 at a pressure of 2,000 lbs. per square inch and a temperature of 1,000 F. All of the burners 12 are operating at capacity, firing the furnace 10 with pulverized coal at temperatures of the order of 2,900 F. The sets of recirculated gas flow regulators 120 are set so as to deliver sufiicient recirculated gas through the openings 152 between the tube portions 13' at the noses of the arches 16 and 13 to mix with the combustion products coming directly from the burners 12 and reduce the temperature of the gases within the mixing chamber 20 to a value of the order of 2,200 F. The gas flow regulators 08 at the outlets of the reheater gas passes 30 and 32 are set so as to maintain a reheat temperature of l,000 F. at the inlets of the last two stages of the turbine 74. In the interests of the optimum cycle efficiency these dampers are so set that they will not give any substantial reheat temperature above the optimum 1,000 F. The entire remainder of the gases flow over the intermediate superheater section 56 and the banks of economizer tube 76 and 78, and the gas fiue regulators 100 at the exit of the superheater gas pass 26 are still open, or are only closed partially sufiiciently to insure the attainment of the reheat temperature of 1,000 F. Under these conditions the superheat temperature of the steam issuing from the headers 46 or 58 might be more than the optimum superheat temperature of 1,000 F. at the inlet of the high pressure stage 72 of the turbine. In that event, the attemperators 50 and 62, preferably of the spray type, are operated automatically from the indications of final steam temperature (or superheat), to reduce the superheat steam temperature at the positions of the attemperators to such an extent that the optimum temperature I,000 F. will be attained at the inlets of the turbine. The gas temperatures at the exit of the superheater gas pass 26 will be of the order of 600 F., and the gas temperature at the outlets of the reheater gas passes 30 and 32 will be of the order of 800 F. Under these circumstances the gases flowing through the duct work 102 to the inlet 112 of the recirculated gas system will have a temperature of the order of 700 F. The recirculated gas system introduces at the noses of the arches recirculated gases at a temperature of a value of about 1,600 F. less than the optimum temperature of the gases within the mixing chamber 20 and about 2,200 F. less than the temperature of the gases within the furnace 10.

After a period of such full load operation as that set forth above let it be assumed that the steam demand decreases to 60%, or to 600,000 lbs. of steam per hour. With such a change the firing rate of the burners 12 is automatically and correpondingly reduced. There is a corresponding reduction of gas flow from the furnace into the throat 14, and there is also a substantial reduction in the maximum temperature of the furnace gases from the full load value of 2,900 F. This reduction in gas temperature decreases the requirement for recirculated gas flow for mixture with the burner efiiux gases in the throat 14, and such reduction of the recirculated gas flow is preferably automatically effected by auto matic operation of the recirculated gas flow regulators 1'20. Such a reduction of recirculated gas flow may take place automatically from representations of load or representations of furnace gas temperatures and since there is therefore a marked reduction in the gas mass flow from the mixing chamber 20 and as there is no fully compensating temperature increase in the gases at this position the available heat in the gases for reheating and superheating is markedly decreased. Under these conditions, and with no other influences being operative, the temperature of the steam would drop materially at the outlet of the superheater and, there would be a greater drop in temperature at the outlet of the reheater 33, and a temperature drop the greatest of all, at the outlet header for the low pressure reheater in the gas pass 30. To compensate for the tendencies of the reheaters and superheaters to effect such undesired changes, and, to maintain the optimum reheat and superheat the temperatures, the gas flow over the superheater 56 and through the s pass 26 is reduced by the partial closing of the gas flow regulators 100, to an extent suflicient to cause the gas flow through the passes 30 and 32 to attain the desired reheat temperatures of 1,000 F. Simultaneously, the set of gas fiow regulators 98 at the exit of the gas pass 32 will be partially closed and the similar gas flow regulators at the exit of the gas pass 30 will be partially opened in order to give a differential gas flow control as between the gas passes 30 and 32 to effect differential compensation for the different tendencies of these reheaters to depart from optimum operation in different degrees. The gas flow regulators at the ends of the gas passes 30 and 32 are preferably automatically operable by suitable control apparatus operative through representations of load and reheat temperatures and the difference between reheat temperatures at the exits of the different reheaters.

Simultaneously with the above indicated control, the attemperators 50 and 62 are subject to automatic control from load and final steam temperature to reduce attemperation to such an extent that the optimum superheat temperature 1,000" P. is attained at the inlet of the turbine '74.

It is to be noted that the economizer including the banks of tubes 76 and '78 is situated in a gas pass apart from the reheater gas passes 30 and 32. This arrangement promotes or facilitates the optimum control of the amount of heat absorption in the reheaters over a wide load range where less heat is absorbed (per pound of steam) at full load than at low load. At low loads the gas flow over the reheaters is increased by reducing the gas flow over the superheater, and at the same time the gas flow over the economizer is reduced to reduce its heat absorption thus leaving more heat for the reheaters at low loads. This bypassing of the economizer at low loads partly offsets the increase in heat absorption by the furnace wall vapor generating tubes at low loads and increases the range of steam temperature control.

It will be understood, of course, that when the steam demand increases so that the load is increased from 60% to 100% that the reverse of the above indicated control actions take place.

Thus control of the effectiveness of steam heating is, in part, attained by controlling the temperature of the gases approaching the steam heaters, by regulating the amount of flow of the lower temperature recirculated gases. By this control feature, in combination with the others above referred to the efficiency of the power cycle of the illustrative unit is enhanced by the limitation of the use of spray attemperation to the high pressure steam heated by, and issuing from, the superheater.

The invention has the advantage that it permits a substantial reduction in the size of the furnace which is one of the most expensive parts of many modern high capacity steam generating units. This substantial reduction in furnace size is equivalent to moving the superheater and the reheaters closer to the combustion zone, and this arrangement has a direct enhancement of superheat and reheat control inasmuch as it flattens the superheat and reheat curves and results in a much wider range of steam temperature control, and a substantial reduction in the amount of spray Water used in spray attemperation.

The invention, with its use of recirculated gases as tempering gases to lower the temperature of the gases immediately in front of the steam heaters permits the location of the high temperature steam heating tubes in safe temperature zones where, otherwise, substantially increased expense would be involved by the necessity of using high temperature steel alloys in tubes of unreasonable wall thickness.

By way of completing the description of the remaining details of the unit illustrated in the drawings, reference will be first made to the furnace 10. The burners 12 are preferably arranged in horizontal rows along the wall 154 of the furnace. The burners fire through wall openings provided by bending parts of the furnace wall vapor enerating tubes 156 out of their wall alignment at the positions of the burners. The different horizontal rows of burners are provided preferably with separate pulverizers which deliver streams of primary air and pulverized coal to the conduits 15$-160 leading to the burners. The secondary air is supplied to the burners by their enclosure within the windbox 162 which extends across the front of the furnace and has connected thereto, the side ducts 164 and 166. These side ducts are directly connected with the duct work 168 which is shown in FIG. 1 as leading upwardly from the air heater 104.

Some of the furnace wall vapor generating tubes 156 lead directly upwardly from the lower header 170 and others of these tubes are extensions of furnace wall floor tubes 172 leading upwardly from the header 174 which is shown in communication with the associated header 176 by connection 178. These headers are also in communi-v cation with the water space of the drum 44 through large diameter downcomers, in a manner well known in this art.

Above the right hand furnace wall 154 some of the furnace wall tubes are bent into the furnace and then outwardly again to form the arch 18. Others of these tubes are connected to the header 180. Extending upwardly from this header are other tubes such as indicated at 18 and 182 in FIG. 3. Alternate tubes 18' extend directly upwardly across the recirculated gas outlet of the duct component 1 18, and in this zone the remaining tubes 182 are bent out of their wall alignment in the manner indicated in FIG. 3 so as to provide the recirculated gas outlet openings as indicated by the associated arrows in this figure. These tubes 132 return to wall alignment at a level just above the level of the duct component 118 and then, with the extensions of the other furnace wall tubes 156 all of these tubes continue upwardly for connection with the steam and water drum 44. Some of these tubes extend directly vertically along the inside of the right hand wall 28 of the reheater gas pass, as indicated. At the upper part of the unit they are shown as connected to a header 188 which is in communication of the drum 44 by the circulators 1%.

Others of the tubes extending upwardly past the re circulated gas outlet 113 are bent inwardly in a horizontally inclined arrangement to form the screen 192, disposed across the gas -fiow to the reheater gas passes. Substantially centrally of the unit these tubes continue upwardly in wall formation to present part of the dividing wall 22 between the superheater gas pass 26 and the two reheater gas passes 30 and 32.

Along the Opposite furnace wall 200 and the aligned gas pass Wall 24 there is a similar arrangement of elements, including the furnace wall tubes 202, the header 204, the arch 16, and the screen 206. The extensions of the tube portions forming the screen 206 are interspersed with reference to the extensions of the tube portions forming screen 192 and arranged in Wall alignment therewith to complete the gas pass division wall 22. Some of the vapor generating tubes constituting this wall 22 continue directly upwardly tothe header 210 disposed at the top of the unit and connect with the drum 44 by the circulators 212. Others of the tubes of the wall 22 are bent to the left to define the roof or wall 92 directing the gases from the superheater gas pass 26 to the flue 102 and separating the superheater gas pass from the reheater gas passes 30 and 32 in the zone of the gas flow regulators 98 and 100. The vapor generating tubes of the wall or roof 92 continue in widely spaced vertically arranged tube sections 220 and 222 across the outlet of the reheater gas passes 30 and 32. Thence they continue along the roof of the unit for connection with the header 224 which is in communication with the drum 44 by the circulators 226.

The tubes leading directly upwardly from the header 204 in the zone of the recirculated gas outlet chamber 116 are arranged in a manner similar to that described with reference to the recirculated gas outlet chamber 1118 at the opposite side of the unit, and some of these tubes extend directly upwardly along the gas pass Wall 24 as indicated at 230. They extend to the roof of the unit where they are connected to the upper headers 232 which are in communication with the drum 44 by appropriate circulators 234.

The height of such a high capacity vapor generating and steam heating unit as that described is often within the range of 150 to 200 feet and the weight of the drum 44 and the tubes and headers constituting the reheaters and the superheater at the upper part of the unit may be in excess of 200 tons. It will be therefore understood by those well versed in the art that appropriate heavy steel work is provided to adequately support the unit in all respects. This steel work will include heavy columns along the front and rear walls and Side walls of the unit. It will be also understood that these walls, with their wall tubes, their insulating material arranged exterior of the wall tubes, and their casing components, preferably arranged to make the unit pressure tight, receive lateral support from horizontally spaced beams secured to some of the tubes. These beams form buckstays such as are indicated at 24;}, along the opposite walls of the unit. With respect to the walls, it will be understood that the side walls of the unit are formed in a manner similar to that described, with reference to the front and rear walls. They include side wall headers and connected upright vapor generating tubes generally defining the walls and appropriately connected into the circulation of the unit by headers and suitably circulatory connections.

The pressure tight wall and easing construction for the unit also includes exterior housings for the superheater headers 58 and 66 and the exterior reheater headers such as 136. Such housings are indicated at 242, 24 4 and 246 in FIG. 1.

The housing 244 for the reheater headers 136 also encloses the superheater header 70, and with respect to the reheater headers it is to be noted that these headers are 751 pure 91 slapeeq rs eaqer re n; oqt qnm loqlofiol relatively large diameter. This arrangement with the plural inlet headers and plural outlet headers for the reheater, together with the attendant decrease in the number of tubes connected to each header, enhances the operation of the unit by reducing the pressure drop through the reheaters.

It will be understood from the disclosure in the drawings that the economizer, reheaters, and the superheater sections are each constituted by one or more banks of tubes formed by serially connected return bend tubular sections. The attemperator 50, disposed in the steam flow path between the outlet of the first superheater section 40 and the second superheater section including the banks of tubes 250253, and the attemperator in the line conducting superheated steam from the header 58 to the inlet header 66 of the last superheater section 68 are preferably of the water spray type, such as indicated in the US. Patent 2,550,683 dated May 1, 1951.

The air inlet 26% for the air heater 164 is preferably connected to the outlet of a forced draft fan to cause the movement of air under pressure through the duct 168 to the windbox for the burners 12, and to put the furnace and the remainder of the unit under positive gas pressure.

In the operation of the unit the interposition of burners between the point of recirculated gas introduction and the door of the furnace promotes the maintaining of the slag in molten condition along the refractory fioor 262 of the furnace. The furnace floor is provided with a central slag exit opening 263 leading to the slag pit 264, some of the floor tubes 172 being bent around this opening.

With reference to the circulatory system of the unit it is to be understood that the various vapor generating tubes discharge high pressure vapor and liquid mixtures into the drum 44, and that this drum is provided with effective means for separating the vapor and the liquid. Such an appropriate and effective vapor and liquid separator is indicated by the US. Patent to Rowland et al. 2,289,970. Such vapor and liquid separators associated with other appropriate drum internals, operate to maintain a body of separated liquid in the liquid space of the drum, with dry separated vapor thereabove. Leading downwardly from the water space of the drum 44 is one or more downcomers such as 270 which communicate with the lower headers such as 170, 174 and 176 by appropriate tubular connections.

Whereas the invention has been described with reference to a preferred embodiment of the vapor generating and superheating unit disclosed in the drawings, it is to be appreciated that the invention is not to be considered as limited to all of the details thereof. The invention is rather to be taken as of a scope commensurate with the scope of the sub-joined claims which cover different combinations and subcombinations.

What is claimed is:

1. In a vapor generating, vapor superheating and vapor reheating unit; means including vapor generating tubes defining boundaries of a slag tap furnace, a gas mixing chamber, and three gas passes leading in parallel from the mixing chamber; means firing the furnace with a slag forming fuel at temperatures above the fusion temperature of the incombustible residue; some of said tubes along opposite furnace walls being bent inwardly of the furnace to delineate the inwardly extending arches and the gas mixing throat of the mixing chamber; a gas recirculation system withdrawing gases from a position downstream of at least the main part of one of said gas passes and introducing the withdrawn gases through openings between tubes of said arches into said throat and the mixing chamber; means whereby the rate of introduction of said gases may be varied when the rate of vaporization changes; an economizer and a convection superheater in one of said gas passes; a first vapor reheater in the second of said gas passes for reheating exhaust vapor at one pressure less than the pressure of the superheated vapor; at second vapor reheater in the third gas pass reheating exhaust vapor at a still lower pressure; and means at the outlets of said gas passes for independently controlling the vapor heating effects of the different vapor heaters therein.

2. In a vapor generating, vapor supcrheating, and vapor reheating unit; means including vapor generating tubes defining boundaries of a polygonal furnace, a gas mixing chamber, and three gas passes leading in parallel from the mixing chamber; means firing the furnace at temperar tures above values permissible for superheater contact;

some of said tubes along opposite furnace walls being bent inwardly of the furnace to delineate the inwardly extending arches and a gas mixing throat of the mixing chamber; an economizer and a convection superheater arranged in series as to gas fiow in one of said gas passes; a first vapor reheater in the second of said gas passes; a second vapor reheater in the third gas pass; means at the outlets of said gas passes for independently controlling the vapor heating effects of the different vapor heaters therein; a gas recirculation system withdrawing gases from a position downstream of vapor heating surface in one of gas passes and introducing the withdrawn gases through portions of said arches into said throat and the mixing chamber; and means whereby the rate of introduction of said gases may be varied when the rate of vaporization changes.

3. In a vapor generating, vapor supcrheating, and vapor reheating unit; means including vapor generating tubes defining boundaries of a polygonal slag tap furnace, at gas mixing chamber for receiving the furnace gases, and

three gas passes leading in parallel from the mixing chamber; means firing the furnace with a slag forming fuel at temperatures above the fusion temperature of the incombustible residue of the fuel; some of said tubes along opposite furnace walls being bent inwardly of the furnace to delineate inwardly extending arches and a gas mixing throat of the mixing chamber; means including said arches forming recirculated gas outlets or outlet chambers on opposite sides of said throat; a first vapor reheater in the second of said gas passes; a second vapor reheater in the third gas pass; a gas recirculation system withdrawing gases from a position downstream of at least a substantial part of one of said reheaters and introducing the withdrawn gases through the nose portions of said arches into said throat and the mixing chamber; means whereby the rate of introduction of said gases is varied as the rate of vaporization changes; an economizer and a convection superheater arranged in series as to gas flow in one of said gas passes; and means at the outlets of said gas passes for independently controlling the vapor heating effect of the different vapor heaters therein; the arch delineating tubes continuing upwardly of the arches as inclined screens across the inlets of said passes and then upwardly from the screens as wall aligned tubes delineating an upright wall separating the superheater and economizer gas pass from the gas passes for the reheaters.

4. In a vapor generating and heating unit; means including vapor generating wall tubes defining boundaries of a vertically elongated furnace of polygonal horizontal section, a gas mixing chamber having its lower end opening to the upper part of the furnace, and a convection gas pass arranged to receive heating gases from said gas mixing chamber, some of said vapor generating tubes along said walls being bent inwardly out of wall alignment to form an inwardly projecting arch extending across said furnace and delineating at least one side of a throat of substantially reduced cross sectional flow area for movement of gases between said furnace and said gas mixing chamber, said inwardly bent vapor generating tubes and wall aligned vertical vapor generating tubes defining a recirculated gas receiving chamber within said arch, means for firing the furnace for generation and flow of heating gases through said furnace, gas mixing chamher and convection gas pass in the order named; a vapor heater positioned in said convection gas pass; a gas recirculation system arranged to withdraw gases from a position downstream of said vapor heater in said convection gas pass, and means for introducing the withdrawn recirculated gases into said recirculated gas receiving chamber and through said arch to mix with the heating gases generated in said furnace and delivered to said gas mixing chamber to temper the heating gases before said gases enter said convection gas pass; and means whereby the rate of introduction of said recirculated gases may be varied when the rate of vaporization changes.

5. In a vapor generating and heating unit; means including vapor generating wall tubes defining boundaries of a vertically elongated furnace of rectangular horizontal section, a gas mixing chamber having its lower end opening to the upper part of the furnace, and a convection gas pass arranged to receive heating gases from said gas mixing chamber, some of said vapor generating tubes along said walls being bent inwardly out of wall alignment into said furnace to form a hollow arch extending across said furnace and delineating at least one side of a throat of reduced cross sectional flow area for movement of gases between said furnace and said gas mixing chamber, means below the gas mixing chamber for firing the furnace for generation and flow of heating gases through said furnace, gas mixing chamber and convection gas pass in the order named; a vapor heater positioned in convection gas pass; a gas recirculation system arranged to withdraw gases from a position downstream of said vapor heater in said convection gas pass, means forming slotted openings in said arch, and means for introducing the withdrawn recirculation gases to said hollow arch and through said slot-ted openings in said hollow arch into said furnace to mix with the heating gases generated in said furnace and delivered to said gas mixing chamber, said slotted openings being positioned in the innermost end portion of said arch and elongated in the direction of heating gas flow for effective mixing of recirculated gases with said heating gases; and means for regulating the rate of introduction of said recirculated gases into said furnace.

6. In a boiler the combination of an elongated furnace into which fuel is introduced generally at one end and burned therewithin and which is of generally polygonal transvers section at least at its other end and is provided with an outlet at said other end and through which the combustion gases thus generated are conveyed, heat exchange means disposed to be traversed by the gases issu ing from said outlet, a nose baffie extending across the upstream edge of said outlet and projecting into said furnace, said baffie being comprised of a wall extending laterally from one furnace side wall into the furnace to a predetermined distance whereat it is reversely bent and extended back to the plane of said furnace Wall with the laterally extending portions of the wall of the b-aflie being spaced longitudinally of the furnace, wall means extending across the spaced edges of the wall of the baffie and across the ends of the baffle, the walls of the baffie and said wall means being constructed so as to form a gas duct, means for recirculating combustion gases to the furnace including means for introducing said combustion gases into the duct formed by the baffle and said wall means with said baffie being provided with openings disposed to direct these gases into the combustion gas stream passing through the furnace.

7. The combination of claim 6 wherein the walls of said furnace and said nose baffle include fluid cooled tubes, and the openings in said nose baffle are elongated and formed in the spaces between the nose baffle tubes.

References Cited in the file of this patent UNITED STATES PATENTS 2,628,598 Van Brunt Feb. 17, 1953 2,798,464 Seidl July 9, 1957 2,800,114 Chan et al. July 23, 1957 2,815,007 Sprague et al. Dec. 3, 1957 2,869,520 Paul-ison Jan. 20, 1959 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent N0, 3 115 123 December 241 1963 Charles S Smith It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 7,, line 44h for the upside down matten read together with the inlet reheater headers 132 and 134 are column 8 line 7,, for "Rowland" read me Rowand line 22 after "different" insert aspects of the invention including different column 10 line ll after "in" insert said line 28, for "'transvers' read transverse line 59, for "2 8OO 114" read we 2 800 115 e Signed and sealed this 16th day of June 19640 (SEAL) Attest:

ERNEST W. SWIDER EDWARD J. BRENNER Altesijing Officer Commissioner of Patents 

1. IN A VAPOR GENERATING, VAPOR SUPERHEATING AND VAPOR REHEATING UNIT; MEANS INCLUDING VAPOR GENERATING TUBES DEFINING BOUNDARIES OF A SLAG TAP FURNACE, A GAS MIXING CHAMBER, AND THREEE GAS PASSES LEADING IN PARALLEL FROM THE MIXING CHAMBER; MEANS FIRING THE FURNACE WITH A SLAG FORMING FUEL AT TEMPERATURES ABOVE THE FUSION TEMPERATURE OF THE INCOMBUSTIBLE RESIDUE; SOME OF SAID TUBES ALONG OPPOSITE FURNACE WALLS BEING BENT INWARDLY OF THE FURNACE TO DELINEATE THE INWARDLY EXTENDING ARCHES AND THE GAS MIXING THROAT OF THE MIXING CHAMBER; A GAS RECIRCULATION SYSTEM WITHDRAWING GASES FROM A POSITION DOWNSTREAM OF AT LEAST THE MAIN PART OF ONE OF SAID GAS PASSES AND INTRODUCING THE WITHDRAWN GASES THROUGH OPENINGS BETWEEN TUBES OF SAID ARCHES INTO SAID THROAT AND THE MIXING CHAMBER; MEANS WHEREBY THE RATE OF INTRODUCTON OF SAID GASES MAY BE VARIED WHEN THE RATE OF VAPORIZATION CHANGES; AN ECONOMIZER AND A CONVECTION SUPERHEATER IN THE ONE OF SAID GAS PASSES; A FIRST VAPOR REHEATER IN THE SECOND OF SAID GAS PASSES FOR REHEATING EXHAUST VAPOR AT ONE PRESSURE LESS THAN THE PRESSURE OF THE SUPERHEATED VAPOR; A SECOND VAPOR REHEATER IN THE THIRD GAS PASS REHEATING EXHAUST VAPOR AT A STILL LOWER PRESSURE; AND MEANS AT THE OUTLETS OF SAID GAS PASSES FOR INDEPENDENTLY CONTROLLING THE VAPOR HEATING EFFECTED OF THE DIFFERENT VAPOR HEATERS THEREIN. 